In the photolithographic process for manufacturing semiconductor devices and so on, projection exposure apparatus in which a pattern image of a photomask or a reticle (generically called “reticle” hereafter) is exposed onto a workpiece such as a wafer, or a glass plate and the like coated with a photoresist and the like via projection optical system have been used. Then, the resolving power (resolution) required for the projection optical system of the projection exposure apparatus has been increased more and more in order to improve the integration level of semiconductor devices and so on. As a result, the wavelength of illuminating light (exposure light) must be shortened and the numerical aperture (NA) of the projection optical system must be increased.
For example, if an exposure light with wavelength of 180 nm or less is used, it is possible to achieve a high resolution of 0.1 μm or less. However, if the wavelength of illuminating light is shortened, the absorption of light becomes remarkable, and the kinds of glass materials (optical materials) that can be practically used are limited. In particular, if the wavelength of illuminating light becomes 180 nm or less, the practically usable glass material is limited to fluorite only. As a result, the correction of chromatic aberrations becomes impossible in a dioptric type projection optical system. Here, the dioptric type optical system is an optical system which does not contain reflective surfaces (concave reflective mirrors and convex reflective mirrors) with power, but only contains transmissive optical members, such as lens components.
As described above, there is a limit to the allowable chromatic aberrations in a dioptric type projection optical system, and a very narrow band of laser light source is needed. In this case, an increase in the cost of laser light source and a decrease of its output are unavoidable. Moreover, many positive lenses and negative lenses must be arranged in a dioptric optical system to bring the Petzval sum, which affects the curvature of image field, close to 0. By contrast, a concave reflective mirror corresponds to a positive lens as an optical element for converging light, but it is different from a positive lens in that no chromatic aberrations occur and that the Petzval sum takes a negative value (a positive lens takes a positive value in this connection).
In a so called catadioptric optical system constituted by combining a concave reflective mirror and lenses, the above characteristic of the concave reflective mirror is best used to the maximum in an optical design and good correction of aberrations beginning with the chromatic aberrations and the curvature of image field are possible in spite of its simple construction. However, the manner in which an incident beam and an emergent beam are separated for a concave reflective mirror is point of greatest difficulty, and various techniques for this separation have been proposed.
For example, Japanese Laid-Open Application No. 8-62502 (U.S. Pat. No. 5,861,997) discloses a catadioptric optical system which is a catadioptric optical system using an exposure region (off-axis visual field) free of an optical axis in a projection exposure apparatus and is of a type wherein intermediate images are formed twice on the way of the optical system and the separation of beam is spatially conducted in the vicinity of the intermediate images.